Hx-Push-Url headers

[dsyer]

The current version of HTMX uses "Hx-Push" so maybe that should be fixed (it still supports the older value used here). But I wonder why we use it at all? It's ugly to see the servlet request and response being used in controller methods, and makes them hard to test, and HTMX supports setting the push url in the HTML which seems like a good idea to me. Am I reading that right?

Fixed by #5 and #12